feat(PROJ-30): Xapian → Manticore Search Migration

- internal/index/manticore.go: ManticoreTenantManager + manticoreIndex (RT-Indizes, CGO-frei)
- internal/index/index.go: TenantIndexer Interface (Xapian + Manticore)
- internal/index/tenant_worker.go: mgr-Typ auf TenantIndexer Interface
- internal/api/server.go: idxMgr auf TenantIndexer Interface
- config/config.go: IndexConfig.ManticoreDSN Feld
- cmd/archivmail/cmd_reindex.go: reindex Subkommando
- cmd/archivmail/main.go: Manticore-Branch + reindex Case
- go.mod: github.com/go-sql-driver/mysql v1.8.1
- update.sh: Manticore auto-install, CGO_ENABLED=0, config.yml migration, auto-reindex

fix(IMAP): TCP-Deadline-Wrapper für steckengebliebene Imports
fix(auth): Email-Claim in JWT für User-Isolation
fix(search): User-Isolation via sess.Email (fail-safe)
fix(ui): Admin-Login Auth-Cache, Logout-Redirect, IMAP-Polling-Resilienz

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/UserNav.tsx

@@ -21,13 +21,15 @@ interface UserNavProps {
 export function UserNav({ username, role }: UserNavProps) {
   const router = useRouter();
 
+  const ADMIN_ROLES = ["auditor", "admin", "domain_admin", "superadmin"];
+
   async function handleLogout() {
     try {
       await logout();
     } catch {
       // ignore logout errors
     }
-    router.push("/");
+    router.push(ADMIN_ROLES.includes(role) ? "/admin/login" : "/");
   }
 
   return (